Remove manual `--cfg docsrs` in `Cargo.toml` metadata

[BD103]

What problem does this solve or what need does it fill?

At the bottom of every public crate is the following:

[package.metadata.docs.rs]
rustdoc-args = ["-Zunstable-options", "--cfg", "docsrs"]
all-features = true

This allows code to check that it is building on docs.rs using #[cfg(docsrs)]. It's frequently used for nightly docs annotations, such as enabling auto-feature-documentation. The --cfg docsrs section of rustdoc-args can be removed, since it is now passed to all crates instead of just the main one being built.

What solution would you like?

Remove --cfg docsrs from the Cargo.toml, and re-evaluate if -Zunstable-options is still necessary.

Additional context

• Add --cfg docsrs to all rustdoc invocations rust-lang/docs.rs#2390
• Native support for docs.rs conditional compilation rust-lang/docs.rs#2389